The present invention relates to a piezoelectric actuator having a bottom electrode attached to a membrane, a thin piezoelectric layer disposed on the bottom electrode, and a top electrode formed on the piezoelectric layer, wherein the bottom electrode extends over the entire bottom surface of the piezoelectric layer, and at least a peripheral portion of a top surface of the piezoelectric layer arid side faces of that layer are covered with an insulating layer. The present invention also relates to a method of producing such an actuator.
More particularly, the present invention relates to a piezoelectric actuator in an ink jet device that is used in an ink jet printer for expelling an ink droplet in response to an electrical signal energizing the piezoelectric actuator. The actuator, when energized, causes the membrane to flex into a pressure chamber, so that the pressure of liquid ink contained in that chamber is increased and an ink droplet is ejected from a nozzle that communicates with the pressure chamber.
The actuator is operated in a flexural deformation mode. This means, that, when a voltage is applied between the top and bottom electrodes, the piezoelectric layer bends in the direction normal to the plane of the layer and thereby causes the membrane to flex in the same direction. As a consequence, the piezoelectric layer must be thin, in the sense that the thickness of the layer is smaller than at least one dimension of that layer in the plane that is parallel to the plane of the membrane surface.
US 2005/275316 A1 and US 2004/051763 disclose actuators of this type, wherein the bottom electrode is formed as a continuous layer on the membrane, which layer extends beyond the edge of the piezoelectric layer. The insulating layer is formed directly on the top surfaces of the piezoelectric layer and the bottom electrode for separating the bottom electrode from an electrically conductive lead that contacts the top electrode from above, through a hole in the insulating layer.
US 2005/0046678 A1 discloses an actuator, wherein the piezoelectric layer extends beyond the edge of the bottom electrode on at least one side where an electrical contact is applied to the top electrode. This configuration assures a certain distance between the bottom electrode and the conductor that contacts the top electrode, and thus prevents the electrodes from being short-circuited inadvertently.